1. Field of the Invention
The invention relates to a circuit and method for the erasure of a non-volatile and electrically erasable memory. The invention relates more particularly to FLASH type memories and also to block-erasable EEPROM type memories.
2. Discussion of Related Art
The erasure of FLASH (and EEPROM) type memories is currently done by tunnel effect (or Fowler Nordheim effect). In FLASH memories, the erasure is done on all the memory cells or at least on a block of memory cells. This type of erasure is also performed in certain types of EEPROMs.
To erase a memory area, all the cells are initially preprogrammed and then, in order to carry out an erasure by tunnel effect, a voltage of 0V (or a negative voltage) is applied to the gates of the storage transistors and a voltage of about 10V (or more) is applied to the sources of the storage transistors, the drains of the transistors being left in a floating state. For EEPROM type memories, the various voltages are applied during a certain period of time calculated so that all the cells are in an erased state. As a result, certain cells that react more quickly than the others have their floating gates positively charged, making the storage transistor always conductive without taking into account the gate voltage. For the EEPROMs, this problem results in a slightly greater expenditure of energy during the programming.
With regard to FLASH type memories, this problem should not arise since FLASH memories are directly controlled by the gate of the storage transistor and have no selection transistors available. The approach that is currently being developed consists in sending a series of pulses to the source with an amplitude of approximately 10V and a duration of approximately 10 ms and checking, after each pulse, to determine if the cells are properly erased. The pulses are sent until all the cells are erased or until the end of a defined number of pulses, up to a maximum of 1000. In general, a sorting is done by the manufacturer so that all commercially distributed memories get erased well before the maximum number of pulses is reached. To correct the phenomenon of depletion caused by the excessively quick erasure of the cells, a partial programming stage comprises a cell-by-cell rectification of the levels of the excessively sensitive cells.
Although pulses of approximately 10V are necessary to deprogram the cells, there is a certain degree of variation resulting, for example, from manufacturing defects or a wearing out of the cell characterized by cells that get erased too quickly or too slowly. In other words, some memories are declared to be defective during manufacture or wear out prematurely which makes them unusable because a single pulse of approximately 10V turns them constantly on or because a series of pulses does not erase them.